(a) Encoders in incremental printing--Most large-format incremental printers use a linear encoder in determining and controlling printhead-carriage position and called "codestrip", tensioned along the scan-axis structure, and an encoder sensor that is assembled on the carriage--with a groove for the strip.
The sensor electrooptically reads markings on the taut strip. Associated electronics generates electronic pulses for interpretation by circuitry in the printer.
Some early tensioned encoder strips were all plastic, adequate for small, desktop printers but not for larger printer/plotter machines. Other early strips were glued to the carriage-supporting "beam" structure, but such a solution gave up the advantages of a separate tensioned strip--including much easier assembly and disassembly, on the assembly line as well as in the field.
Representative work of recent years in codestrip refinement appears in the Wilcox and Arminana documents mentioned above. Such work in electronic interfacing appears in the Majette patent.
(b) Alignment--Accurate readings, and also minimization of noise in operation, require good alignment between the strip and sensor. Maintaining such performance reliably over the life of a product requires avoiding friction and wear--which in turn makes alignment even more important.
In the evolution of large-format printer/plotters, recent developments have tended toward use of these devices to print wider and wider mechanical drawings and posters. Of course these applications require wider-bed printing machines with correspondingly longer codestrips.
Alignment, however, is progressively more difficult for longer codestrips, partly because of the tendencies to sag under the influence of gravity and twist slightly due to very small variations in mounting angle at each end of the strip. A particularly problematic cause of misalignment is vibration in the working environment.
Vibration sources include impacts from nearby industrial construction, heavy motor traffic, elevators within the building and the like. Nevertheless, for codestrips of the type introduced in the Arminana document, alignment has been under good control heretofore in systems having modest overall carriage travel--below about one meter (roughly three feet).
(c) The one-meter barrier--More recently it has been noted that performance for strips spanning about 107 cm (31/2 feet) is acceptable, but only marginally so. A current generation of these machines requires encoder strips with spans of 152 cm and 183 cm (five and six feet respectively). In a machine of this size the associated long dimensions of the strip cause failures in functional-vibration tests, particularly in large-amplitude harmonic movement near the middle of the strip.
This vibration can produce bad readings from the sensor. For instance the counter may miss counting one or more scale graduations on the encoder strip. The result can be significant errors in a printed image.
In cases that are even more serious, vibration causes complete disassembly of the sensor system--as the strip jumps entirely out of the sensor groove. In such cases trained service personnel may be required to restore normal operation.
Damage to the strip can occur, and the sensor too may require repair. To prevent such problems the system is programmed to shut down the carriage servocontrol motor if the sensor system is able to detect that it has lost count of the encoder graduations--as for example if it loses the pulse train completely.
If such a loss of count occurs while the carriage is near either end of the mechanism, and moving rapidly toward that end, this safety override may not have enough time to stop the carriage before it reaches the end bulkhead. Considerable damage to the carriage and other parts of the mechanism can result.
For machines of modest size it is sufficient to provide a mechanical limiter that simply retains the strip within the sensor groove. The limiter and its installation represent undesirable added cost.
This simple solution, moreover, has proven inadequate for a strip over 11/2 m long. Even though retained within the sensor, the strip undergoes oscillations large enough to make sensor measurements erratic and unreliable.
People familiar with this field will understand that the "barrier" suggested in the title of this subsection is not an abrupt step at precisely one meter. Rather the difficulty in achieving satisfactory codestrip arrangements increases progressively over a considerable range from, perhaps, less than one meter to two or possibly three meters. Nevertheless there is a clear qualitative difference, between lengths under one meter and lengths of, say, several meters.
(d) An overconstrained problem--The encoder strip is a rather simple mechanical article, but those skilled in the art will recognize that this seeming simplicity may be very deceptive. The strip interacts in subtle ways with several different complex components of the system.
As a result, it is not at all obvious how to overcome the difficulties outlined above. Some of the more-evident candidate solutions are impractical, due to certain persistent constraints.
The progressively larger machine formats, even below the one-meter barrier suggested earlier, have called for greater tension in the strip. Beyond that barrier, simple increase of tension in the strip is unacceptable.
One reason is that higher tension could potentially introduce safety concerns. Another reason is that higher tension in the strip can cause small twists and other irregular deformations in the associated mechanism. Even if microscopic, such interference with the straightness and structural integrity of the guide-and-support rods and beam can throw off the positional calibration of the whole carriage drive system.
Such potential damage can be difficult to detect, and the design cost of reevaluating the entire mechanical system for such potential damage is in itself severe. If found, such a problem can be compensated only by strengthening the entire structure. Beefing up the mechanism in that way, in turn, would entail additional weight and cost.
Another complication is that addition of stiffening elements or any other attachment to the strip itself would be extremely awkward, since the sensor groove is very narrow. Of course it is important not to add anything to the strip, or next to it, that might pose even greater risk of damage than the strip itself poses--that is to say, catastrophic failure modes must be evaluated as carefully as routine operation.
Thus a supporting ledge below the codestrip (reasonably remote from the moving sensor) might be useful, although costly, but it would not resolve the problem of the strip moving upward. A "ceiling" strip immediately above the codestrip, to correct that deficiency, does not appear practical since the encoder sensor--moving at high speed--could strike such a component.
It has been suggested to return to the approach of using adhesive to secure a strip to a solid beam structure. As mentioned earlier, however, that approach has associated inefficiencies and high costs. Such a beam-mounted encoder strip is also difficult to install and remove.
Using small screws or bolts to fix a thin metal strip along the base would be even more undesirable. The assembly time required to thread in several screws is a significant cost in terms of modern production engineering. A separate rigid structure--to be bolted into place on the beam--would be still more impractical.
Still another difficulty of earlier codestrip designs relates to the dimension stack. The dimension stack is the group of geometrical dimensions that must be algebraically added to calculate the relative position between two specified parts.
Every dimension has a tolerance. If the number of dimensions is large--i. e. if the dimension stack is "long"--the tolerance can become very large, which is very undesirable.
The pertinent parts in this case are the encoder strip and sensor, and the most problematic dimension is vertical alignment between the graduations and the sensor. For use of standardized parts and good performance, clearance between the top of the strip and the top of the sensor groove is only about two millimeters; and the graduations are roughly just four millimeters tall.
Accordingly in one common failure mode the codestrip strikes the upper end of the groove. In another, as mentioned earlier, the downward-moving codestrip entirely leaves the groove.
Making the groove substantially taller would result in greater noise levels in the electronics system. It would also implicate still further problems of mechanical alignment between parts.
The encoder dimension stack for large-format printer/plotters is in fact undesirably lengthy. It is long primarily because of the tensioned mounting system--and also because the codestrip itself in these wide-bed systems is literally long, leading to large variations in vertical position at each point along the strip.
In particular, the stack for the vertical relationship between the encoder-scale graduations and the immediately adjacent sensor includes the mounting tolerances within the sensor, and tolerances of the sensor mounting to its carriage. Next the stack continues through the carriage, and the carriage bushings, to the rods--then the beam, then the codestrip, and finally tolerances within the strip to the scale graduations.
As a result, variation between machines, as to the vertical sensor-to-scale alignment, is very large. Mounting and configuration of the strip itself, however, accounts for much of this variation.
Finally, an ideal solution should be one that is amenable to routine incorporation into not only 11/2 to 2 m printers but also into both smaller and larger systems. For instance, a solution should be usable in 107-cm units previously described as "marginal" in encoder-strip performance, and also in 3 m or 7 m systems.
It would be an added bonus to find a solution that could be implemented in a retrofit mode for any smaller systems installed in especially problematic (high vibration) environments. As this discussion shows, the codestrip problem is a particularly knotty one that defies easy solutions.
(e) Conclusion--Codestrip instabilities have impeded the extension of uniformly excellent incremental printing to images well over a meter wide. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.